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THE TRANSPORTATION OF GASES                       185
This gives the maximum theoretical draft with no flow taking place. The actual draft obtained is about 20 per cent less, due to chimney friction, and head required to create velocity of gases.
Kent's draught formula is
rr  _   /0.3hp.\ 2
u " \~E.....7
E = A -0.6\/A
Where E = effective area of chimney, A = actual area (cross-section) of chimney, // = height at top in feet above grates and horsepower = total connected boiler horsepower. Based on a maximum combustion of 5 Ib. coal per rated boiler horsepower per hour, this formula may be modified to
rr   _    /O.QGgA  2
where W « total pounds coal fired per hour under connected boilers. This form is more suitable for high rating conditions. See p. 31 et seq.
Mechanical Draft.—From the preceding discussion, it is evident that for high rateK of combustion the stack becomes impracticably high or the sensible heat loss due to high flue temperature becomes too large for economy. To mitigate this, mechanical or artificial draft of some form may be employed.
The theoretical pressure produced by a revolving fan wheel is given by Murgue as
_ .-_
where // « maximum pressure difference, between fan suction and discharge, in feet of air; U = velocity of fan blade tips, feet per second; and g — 32.2, acceleration clue to gravity. Air pressure in inches of water column is generally referred to in blast and draft.
.      SH
h = , .• 144p
where h = inches of water; $ = weight of 1 cu. ft. air at 75°F. (usual room temperature); or 0.074495 11).; and p = pressure of 1 in. water column in pound per Hquare inch, or 0.3061 Ib.
U =   6Q
where r =-• radius to tip of blade in inches and n = r.p.m.    Hence
h =  ('*%*) *S = C(r»)'S
The draft loss through a B. & W. boiler with standard baffling, clean, varies from 0.10 in. or 0.15 in. at rating, to 0.29 in. to 0.45 in. at 200 per cent rating and 0.54 in. to 0.05 in. ut 300 per cent rating. The loss in flues of fairly uniform cross-neotion is given as 0.1 in. per 100 ft. straight flue, and 0.05 in. for 90-deg. bends. But in the irregular flue shapes often necessary, double these values are not infrequent.l The draught loss through economizers varies from 0.25 in. to 0.70 in. The sum of all those losses gives the required chimney draft. The diameter is computed from the weight of coal burned. With stokers using blast it is necessary only to allow 0.1 in. draft over the fire at maximum capacity, as the only function of the stack is to draw off the gases without pressure in the furnace, the resistance of fire bed and stoker being overcome by the blast pressure.
Brick Chimneys.—The red-brick chimney cannot be less than 0.7 ft. thick at the thinnest portion; in other respects it is designed for stability against a wind pressure of 50 Ib. per square foot and against crushing. The walls, therefore, are
i H. C. MEYER, JR. "Steam Power Plants," McGraw-Hill Book Co., 1912.